Most companies and organisations rely on the public power supply grid for the supply of electrical power to machinery, lighting, laboratory and office equipment. In many modem buildings there is generally a back-up power supply, typically a diesel engine powered generator, to supply internal power for the building in the event of power failure. However sensitive electrical equipment such as computer systems and telecommunications equipment are frequently provided with a dedicated uninterruptible power supply (UPS) to ensure that there is no interruption to the supply of electrical power that may result in the loss of valuable information held in volatile memory. A UPS is also intended to regulate the supply of electrical power to an appliance so as to minimise fluctuations in the voltage supplied to the appliance.
A typical prior art UPS is connected to the AC power grid and converts the incoming AC voltage to a bipolar DC voltage across two storage capacitors. This bipolar DC voltage is then reconverted to a regulated AC output voltage. The back-up battery voltage is converted to a high bipolar DC voltage and feeds into the storage capacitors in the event that the input AC voltage falls below an acceptable level. There are a number of disadvantages with such conventional UPS technology. Firstly, there are inefficiencies and power losses associated with the need to convert the voltage twice, from AC to DC and vice versa. Secondly, the two storage capacitors are quite bulky because they need to filter out the 50/60 Hz voltage ripple. They are also costly because they need to have high ripple current capability, high capacitance and high voltage characteristics. Further, this type of UPS circuit does not readily lend itself to a high level of integration.